mcrypt removed from PHP 7.2 core

[buskamuza]

Important: Use libs-upgrade branch as base for this task and as target for the PR.

Investigate possible solutions.

Options:

1. use sodium
2. use mcrypt_compat lib
3. require PECL installation of mcrypt
4. Other

Design document - https://github.com/magento-engcom/php-7.2-support/wiki/HLD---Removing-mcrypt-and-adding-libsodium

[buskamuza]

Note ready for work yet. Requires investigation of which library to use.

[buskamuza]

In case of moving to Sodium:

• Couple framework encryption interfaces which should be reimplemented
• magento/zf1 usages must be adjusted
• A lot of usages in 3rd party libraries, compatible versions existence must be checked

[giabmf11]

We have two opitons:

1. Use the mcrypt extension compatible with PHP 7.2 for the next release since it won't require much development time. Mcrypt isn't being maintained anymore so we should integrate a new library at some point but it will take much more time and introduce BC issues.
2. Use the libsodium bundled extension inherently supported by PHP 7.2 and refactor our code to integrate the new code.

@piotrekkaminski I sent you an email asking your opinion. Let me know if you want to set up a meeting.